Magnetic Resonance Imaging (MRI) is a commonly used diagnostic tool to image soft tissues in the human body. MRI systems use strong magnetic fields. The strong magnetic fields can attract ferromagnetic objects with significant force and torque while the pulsed gradient and radio frequency (RF) fields may induce currents that can produce significant heating in metallic objects. For example, Sommer et al. (Sommer T, Vahlhaus C, Lauck G, von Smekal A, Reinke M, Hofer U, Block W, Traber F, Schneider C, Gieseke J, Jung W, Schild H: MR imaging and cardiac pacemakers: in-vitro evaluation and in-vivo studies in 51 patients at 0.5 T Radiology 2000, 215(3):869-79.4) demonstrated the potential for induced heating as much as 23.5° C. at specific absorption rate (SAR) levels of only 1.3 W/kg in a 0.5 Tesla MRI unit.
A desirable patient for an MRI procedure (e.g. MRI scan) is one who does not have any metallic medical devices implanted in his or her body that would thus avoid interaction with the magnetic and RF fields associated with the MRI scan. However, a significant percentage of potential patients for MRI have implanted medical devices such as pacemaker leads, stents, clips, plates, and joint prostheses. Even if the metal in the implanted device is non-ferromagnetic, the RF field associated with the MRI procedure can lead to harmful heating effects as a result of the RF induced currents in the metal. Hence there is a need for metal alloys for medical devices that are less susceptible to such harmful heating effects. Conventional solutions to the MRI interaction problem have generally relied on coatings or resonator technologies external to the implanted device, such as copper windings.
MRI can also be used for imaging of implanted medical devices during a medical procedure. Medical devices are generally defined as products used for medical purposes in patients, in diagnosis, therapy or surgery. If applied to the body, the effect of the medical device is primarily physical, in contrast to pharmaceutical drugs, which exert a biochemical effect.
Examples of imaging of implanted medical devices during a medical procedure include real-time imaging while placing a stent, using a catheter or other medical device in the human body. It may also include post-implant imaging of the implanted stent or other medical device during the life of the patient. In such cases the medical implant may not be visible clearly in MRI resulting in imaging artifacts such as areas of poor or no contrast, or in some cases even not providing visibility through the device. An example is struts of a stent which do not allow for any image of the tissue or blood vessel between the struts, but the entire stent images without any difference between the strutted and the non-strutted region. Hence there is also a need for metal alloys for medical devices that are less susceptible to such imaging artifacts and poor visibility.
The availability of medical devices having enhanced MRI compatibility devices could enable their extension and incorporation into a suite of medical instruments (e.g., guide wires, catheters, needles, etc.) that would facilitate MRI interventional procedures without the need to examine the patient for possible different types of contraindications that would prevent a person from being examined with an MRI scanner. Such procedures could also replace more traditional fluoroscopic procedures, thereby minimizing the patient's and the physician's exposure to harmful radiation.